Sir,

I guess its like an added security. 

A property eventhough mortgaged, is always vulnerable to unpredicable forces of nature. A small earthquake can turn a tall & sturdy building into rubbles in less than a minute, in which case, the bank's security suddenly ceases to exist. The guarentee is required to compensate such situations.

Anyways, the guarantee will not invoked unless there is some default in repayments.